It has been recognized in the art that the properties of aluminum matrix alloys could be improved in one or more important respects by dispersing throughout the matrix a dissimilar material having little or no solubility in the metal matrix. For example, graphite dispersed in aluminum improves the wear resistance thereof. Graphite, normally speaking, is insoluble in and immiscible with an aluminum melt and would be rejected from such a melt. The problem is solved in accordance with U.S. Pat. No. 3,885,959 by coating the surface with nickel, a metal which is wetted readily by molten aluminum thereby permitting ready dispersal of graphite particles in the aluminum matrix. Other dispersed solid particulate materials can improve the stiffness of modulus or other mechanical properties such as strength, etc. of composite materials having a matrix of aluminum or aluminum alloys. Dispersed silicon carbide is an example of a material which can improve the physical properties of aluminum or aluminum alloys. Other materials when dispersed in fibrous form can improve the strength of aluminum. U.S. Pat. No. 4,012,204 describes the production of fibrous compositions infiltrated with an aluminum lithium alloy. Articles such as
(1) D. Webster, "Effect of Lithium on the Mechanical Properties and Micro-structure of SiC whisker Reinforced Aluminum Alloys," Metall. Trans. A, Vol. 13A, 1982, pp. 1511-1519; and PA1 (2) R. A. Page and G. R. Leverant, "Relationship of Fatigue and Fracture to Microstructure and Processing in Al.sub.2 O.sub.3 Fiber Reinforced Metal Matrix Composites," Fifth International Conference on Composite Materials, TMS-AIME, Proceedings, 1986, pp. 867-886. PA1 (1) Solid-state or semi-solid-state consolidation; PA1 (2) Pressure infiltration or squeeze casting; and PA1 (3) Casting.
also deal with the problem.
In the production of composite materials having a metal matrix, three production methods have been recognized as follows:
Of the above-mentioned three methods, the casting method is the simplest but has been investigated the least. The casting method involves merely the mixing of molten matrix alloy and the reinforcing material at temperatures above the liquidus temperature of the matrix alloy. Once mixed, the molten alloy containing the suggested reinforcing material is solidified by casting in a mold or in the melting crucible.
The casting method has a cost advantage compared to the other methods mentioned and is adaptable to the production of large ingots. The problem with the casting method has been that difficulties have been encountered in wetting the reinforcing material with the molten matrix metal or alloy. Unless wetting of the reinforcing material is effected, rejection from the melt occurs.